Nuon Chea, one of three former Khmer Rouge top leaders on trial in Cambodia for crimes against humanity (AP)

The former deputy leader of Cambodia's Khmer Rouge, accused over the deaths of 1.7 million people, has claimed the regime was acting in the country's best interests.

Nuon Chea told a UN-backed tribunal that it was failing to consider the complete story behind the Khmer Rouge. The tribunal is trying him and two other Khmer Rouge leaders for crimes against humanity and other charges of atrocities committed under the 1970s regime.

All three say they are innocent. Nuon Chea was the chief ideologist for the communist movement and its number two behind Pol Pot. Nuon Chea did not directly respond to the horrors described by prosecutors and instead gave a political history of the Khmer Rouge and Cambodia, insisting his role was patriotic and blaming neighbouring Vietnam for much of the country's troubles.

"I had to leave my family behind to liberate my motherland from colonialism and aggression and oppression by the thieves who wish to steal our land and whip Cambodia off the face of the earth," he said.

"We wanted to free Cambodia from being a servant of other countries and we wanted to build Cambodia as a society that is clean and independent without any killing of people or genocide."

Prosecutor Andrew Cayley said that like Pol Pot, the defendants exercised life-and-death authority over Cambodia while in power.

"The accused cannot credibly claim they did not know and had no control over the crimes that occurred" when they ruled what they called Democratic Kampuchea, he said.

In their opening statements, prosecutors described a litany of horrors, saying the Khmer Rouge sought to crush not just all its enemies, but seemingly the human spirit.

Most people were forced to work on giant rural communes and deprived of any private life. Forced marriages took the place of love, and dissenters were dispatched to so-called "killing fields."

"Here in Cambodia, a unique opportunity has been given ... to set a powerful example, and to send a strong warning from the past to the future so that human beings everywhere can rightfully expect to live in peace under the law," Mr Cayley said.